1. Field of the Invention
This invention relates to a method for removing sulfur dioxide from a gas containing the same. Specifically, this invention allows for virtually complete recovery of sulfur dioxide in a sodium system absorption reaction which may be represented as follows: EQU Na.sub.2 SO.sub.3 + H.sub.2 O + SO.sub.2 .revreaction. 2NaHSO.sub.3 EQU 2naHSO.sub.3 .revreaction. Na.sub.2 S.sub.2 O.sub.5 + H.sub.2 O
2. Description of the Prior Art
Contemporaneous with the sudden increase in public awareness of the necessity for maintaining high environmental quality standards in the United States, numerous apparatus and methods have been devised for improving the environmental quality of the air we breathe. Not surprisingly, industry has also recognized that by "cleaning" their exhaust gases prior to releasing them into the atmosphere, not only is air pollution reduced, but also useful by-products are oftentimes recovered. One of the most notorious of such air pollutants is sulfur dioxide, and numerous prior art patents disclose means and methods for recovering sulfur dioxide from exhaust gas streams. Such prior art methods generally disclose the use of a sodium system for removing and recovering the sulfur dioxide gas. However, for successful, efficient operation of these processes, relatively static gas flow, sulfur dioxide content and chemical solution concentrations are required. Of course, in practice such constancy is virtually unobtainable, necessarily resulting in the use of considerable and complex instrumentation and storage of scrubbing chemicals.
One such prior art process for recovering sulfur dioxide from exhaust gases is disclosed in U.S. Pat. No. 3,485,581. The process disclosed therein is basically two-step and comprises first contacting the exhaust gas with an aqueous solution of the sulfite of a metal selected from the group consisting of alkaline metals and alkaline earth metals in a reaction zone at a temperature between 100.degree. F. and 230.degree. F. to produce an aqueous bisulfite solution, and then passing the metal bisulfite solution through a desorption zone maintained at a temperature of between 300.degree. and 400.degree. F. to decompose the metal bisulfite into metal sulfite, sulfur dioxide and water. Not only would this process be extremely expensive in commercial operation by virtue of the high reaction temperatures required, but also it is only capable of removing 90-95% of the sulfur dioxide from the exhaust gas.
Another process for recovering sulfur dioxide from exhaust gases using an aqueous solution of sodium, lithium or beryllium sulfite is disclosed in U.s. Pat. No. 3,607,037. According to the disclosure of that patent, sulfur dioxide and waste gas are reacted with the sodium sulfite and aqueous solution to form an aqueous solution of sodium sulfite and sodium bisulfite. The sodium sulfite is separated, and the sodium bisulfite solution is heated to produce sodium sulfite and recoverable sulfur dioxide.
However, this process has not proved to be entirely satisfactory, for, like the process discussed in the immediately preceding paragraph, this process must also be conducted at elevated temperatures. Furthermore, the process of this invention must also be conducted at superatmospheric pressures. Obviously, these operating conditions necessarily increase the complexity and the cost of this process. Notwithstanding the complexity of this process, only about 85% of the sulfur dioxide present in the exhaust gas is removed.
Yet another process for the removal of the sulfur dioxide from gas streams is disclosed in U.S. Pat. No. 3,653,812. Much like the prior art methods already discussed, the process of this invention must also be conducted at elevated temperatures and carefully controlled pressures. Furthermore, no more than about 90% of the sulfur dioxide content of the flue gases may be recovered through the use of this process.
Thus, it is apparent that there is a need for a process by which substantially all sulfur dioxide may be removed from exhaust gases. While such a process should be capable of removing up to 99% of the sulfur dioxide content of the exhaust gas, it must also be capable of economic operation. Accordingly, such a process should be capable of being run at substantially atmospheric pressure and ambient temperature. Additionally, such a process should require minimal modification as the sulfur dioxide content of the incoming gas varies.